Wells drilled for the production of oil and gas are commonly finished by cementing one or more metallic casing strings in the borehole. Additional casing strings may be used depending on the surrounding formation and particular properties of the surrounding formation (e.g., formation porosity, formation hardness, flow rate, etc.). For example, a dual casing system may be employed with two concentric metallic casing strings. The two concentric casing strings may be cemented with a layer of cement located in the annular space between the casing strings.
In some circumstances, such as after a well is no longer commercially viable, the well can be abandoned. To satisfy governmental regulations for abandoning a well, one or more permanent barriers may be created inside the well to isolate the well from the surrounding formation. A particular length of the casing string may also be removed prior to filling the well with a cement plug to seal the well. One method of removing a casing string from a well includes the delivery and use of a section mill having multiple blades. The section mill may be coupled to a drill string that is tripped into the well and controlled by a drill rig at the surface.
The blades of the section mill may be in a retracted or inactive state when tripped into the wellbore. The drill string allows placement of the section mill at the desired location in the well, and the blades can be expanded to an active or deployed state through the use of hydraulic fluid provided through the drill string. By rotating the section mill, the expanded blades can remove portions of the casing string and cement in the well.
Where a well has two, concentric casing strings, milling of the dual-casing well can be performed by tripping one section mill into the well to mill the smaller diameter inner casing. That section mill can then be tripped out of the well and a section mill with larger blades can be tripped into the well and located at the same location in the well and for milling the larger diameter outer casing.